Length
of Span.		Width and Thickness of Timber.
6 by 8 inches.6 by 9 inches.6 by 10 inches.6 by 12 inches.
Feet.Pounds.Pounds.Pounds.Pounds.
102,8002,6924,5006,480
122,4003,0423,7505,400
142,0582,6043,2164,632
161,8002,2802,8084,050

A stick of timber twenty feet between supports, will bear a load in its center only one half as great as a timber of the same dimensions, ten feet between supports. Thus four timbers six by twelve inches, in a span of sixteen feet, would bear a load of eight tons; in a twelve foot span, the same timbers would support a weight of nearly twelve tons.

BRACES AND TRUSSES.

Fig. 275.—A Simple Form Of Bridge Span.

Fig. 276.—A Stronger Span.

The above is the initial strength of the timbers which support the weight of the superstructure, and any load that it may have to sustain. But in bridge building these timbers are reinforced by trusses or braces, which add greatly to the sustaining power of the bridge.

[Figure 275] shows the simplest form of a self-supporting bridge, which will answer for spans of from ten to fifteen feet in length. The braces, c, c, reach from near the end of the sill to about four feet above the center. The truss rod, d, is one inch in diameter for short bridges up to two inches for longer spans; it is provided with an iron washer at the top. The rod passes through the sill, and a cross-sill, e, which passes under the main sills, thus adding firmness to the whole structure. Logs, f, f, are placed against the ends of the sills to keep them in place, and where the wheels will first strike them instead of the floor plank, thus greatly equalizing the pressure. [Figure 276] represents a modification of the above. The two truss-rods and braces give the structure greater strength and solidity, adapting it for spans eighteen feet in length. For the latter length, sills should be of good material, ten inches wide and fourteen inches deep, with three middle sills of about the same size.